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Welcome to the forum! Sorry, I can't answer your question about which oil is best. I just wanted to be the first to welcome you to the forum. If I had to pick between the two oils you mentioned I would take the last one you mention for a high mileage car.

I agree. How hard the transmission shifts is controlled by the modulator valve on the transmission. If you have a massive vacuum leak it could account for the poor running and the hard shifting. I would check the vacuum lines and check the intake manifold for leaks. \\

The gas would have to be pretty bad to cause this and with the tank cleaning and filter, are you saying the gasoline is old?? The injector clicking is a good sign but of course it doesn’t guarantee they are functioning at 100%. Short of having them cleaned professionally, a can of Techron or similar can’t hurt and may help. Does disconnecting the MAF help the idle? Readings from the MAF, tps, ignition timing, inlet air temperature, knock counts, O2 cross counts, fuel integrator and block learn, may help track down what is going on. Essentially we need to see if something looks out of range, especially when it is acting up